Cardiac myocytes stimulated with so-called paired, coupled, bi-geminal or intercalated pacing stimulation produce enhanced mechanical function on subsequent depolarizations of the heart. Herein, this type of cardiac pacing therapy is referred to as extra-systolic stimulation (ESS) which refers to delivery of cardiac pacing therapy soon after either an intrinsic or pacing-induced systole. The magnitude of the enhanced mechanical function is particularly dependent on the timing of the extra systole relative to the preceding intrinsic or paced systole. When correctly timed, an ESS pulse causes depolarization of the heart but the attendant mechanical contraction is absent or substantially weakened. The contractility of the subsequent cardiac cycles, referred to as the post-extra-systolic beats, is increased as described in detail in commonly assigned U.S. Pat. No. 5,213,098 issued to Bennett et al., incorporated herein by reference in its entirety.
The mechanism of ESS is thought to be related to the calcium cycling within the myocytes. The extra systole initiates a limited calcium release from the sarcolasmic reticulum (SR). The limited amount of calcium that is released in response to the extra systole is not enough to cause a normal mechanical contraction of the heart. After the extra systole, the SR continues to take up calcium with the result that subsequent depolarization(s) cause a large release of calcium from the SR, resulting in vigorous myocyte contraction.
As noted, the degree of mechanical augmentation on post-extra-systolic beats depends strongly on the timing of the extra systole following a first depolarization, referred to as the extra-systolic interval (ESI). If the ESI is too long, the ESS effects are not achieved because a normal mechanical contraction takes place in response to the extra-systolic stimulus. As the ESI is shortened, maximal stroke volume effect, among others, occurs when the ESI is slightly longer than the physiologic refractory period. An electrical depolarization occurs without a mechanical contraction or with a substantially weakened added contraction. When the ESI becomes too short, the stimulus falls within the absolute refractory period and no depolarization occurs.
The above-cited Bennett patent generally discloses a post-extra-systolic potentiation stimulator for the treatment of congestive heart failure or other cardiac dysfunctions. A cardiac performance index is developed from a sensor employed to monitor the performance of the heart, and a cardiac stress index is developed from a sensor employed to monitor the cardiac muscle stress. Either or both the cardiac performance index and cardiac stress index may be used in controlling the delivery of ESS stimulation. Prior non-provisional U.S. patent application Ser. No. 10/322,792 filed 28 Aug. 2002 and corresponding PCT application (publication no. WO 02/053026) by to Deno et al., which is hereby incorporated herein by reference in its entirety, discloses an implantable medical device for delivering post extra-systolic potentiation stimulation. ESS stimulation is employed to strengthen the cardiac contraction when one or more parameters indicative of the state of heart failure show that the heart condition has progressed to benefit from increased contractility, decreased relaxation time, and increased cardiac output. PCT Publication WO 01/58518 by Darwish et al., incorporated herein by reference in its entirety, generally discloses an electrical cardiac stimulator for improving the performance of the heart by applying paired pulses to a plurality of ventricular sites. Multi-site paired pacing is proposed to increase stroke work without increasing oxygen consumption and, by synchronizing the timing of the electrical activity at a plurality of sites in the heart, decrease a likelihood of development of arrhythmia.
As indicated in the referenced '098 patent, one risk associated with ESS stimulation is arrhythmia induction. If the extra-systolic pulse is delivered to cardiac cells during the vulnerable period, the risk of inducing tachycardia or fibrillation in arrhythmia-prone patients is high. The vulnerable period encompasses the repolarization phase of the action potential, also referred to herein as the “recovery phase” and a period immediately following it. During the vulnerable period, the cardiac cell membrane is transiently hyper-excitable. Therefore, although the property of ESS has been known of for decades, the application of ESS in a cardiac stimulation therapy for improving the mechanical function of the heart has not been realized clinically because of the perceived risks.
In delivering extra-systolic stimulation for achieving mechanical enhancement of cardiac function on post-extra-systolic beats, therefore, it is important to avoid extra-systolic intervals that produce exaggerated shortening of the action potential duration and increased dispersion of the action potential duration and refractoriness. When securely delivered, the mechanical effects of ESS may advantageously benefit a large number of patients suffering from cardiac mechanical insufficiency, such as patients in heart failure. Hence, a method for controlling the timing of the extra-systolic stimuli during extra-systolic stimulation is needed that avoids increased risk of arrhythmias while providing desired beneficial effects of ESS therapy.